What's it doing?
Ordered by how often each cause turns out to be the answer across our five metros.
Running, but the air isn't cold
the most common call
Likely causes, most common first
- Dirty filter. Restricts airflow; the evaporator coil can't absorb heat and eventually freezes. In Houston and Miami humidity this happens faster.
- Thermostat. Set to FAN not COOL, or a wiring fault so the condenser never gets the call.
- Low refrigerant. Systems don't consume it — low means a leak. Recharge without repair is temporary and R-410A is expensive now.
- Dirty condenser coil. Phoenix dust and LA cottonwood pack fins solid; the unit can't reject heat.
- Failed run capacitor. Hums, doesn't start. Heat kills capacitors — Phoenix rooftop units go through them fastest.
- Compressor. Least common, most expensive. Diagnosed by amp draw and pressures.
Check before you call
- Thermostat on COOL, setpoint 3°F under room temp.
- Filter: if it's gray, replace it.
- Outdoor fan spinning? Coil packed with debris? Clear two feet around it.
- Ice on the lines? Shut it off, FAN only two hours, then call — a frozen coil hides the real fault.
What the tech checks
- Temperature split across the coil (15–20°F healthy).
- Static pressure and airflow before blaming refrigerant.
- Superheat/subcooling — the real charge test.
- Capacitor µF against rating; >10% low is replace.
Typical: capacitor $150–$400 · coil cleaning $100–$400 · leak repair + recharge $400–$1,500 · compressor $1,500–$3,000+ (see repair-or-replace below).
Won't turn on at all
no fan, no hum
Likely causes, most common first
- Tripped breaker or blown disconnect fuse. Reset once. Trips again → a short. Stop.
- Thermostat power. Dead batteries or a blown 3-amp fuse on the air handler board.
- Condensate float switch. Pan full, switch cuts power. Very common in humid metros.
- Contactor. Pitted or welded contacts.
- Control board. No 24V call being sent.
Check before you call
- Both breakers — outdoor unit and air handler. Reset once only.
- Thermostat screen blank? Batteries first.
- Standing water in the pan → clear the drain line.
- Outdoor disconnect pull-out seated?
What the tech checks
- 24V at thermostat, air handler, contactor coil.
- Contactor contacts and coil.
- Board fuse and outputs.
- Drain line and float switch.
Typical: drain clearing $75–$250 · contactor $150–$350 · control board $300–$800 · diagnostic $75–$150, usually credited.
Cooling, but weakly
runs all day, never catches up
Likely causes, most common first
- Airflow. Filter, closed registers, collapsed flex duct, slow blower.
- Dirty evaporator coil. Years of bypass dust.
- Marginal charge. Slightly low, not empty.
- Duct leakage. 20–30% loss is normal in older Dallas and LA homes with attic ducts.
- Aging system. A 15-year-old unit delivers 70–80% of nameplate.
Check before you call
- Filter, again.
- Every supply open, every return clear.
- Feel attic ducts for cold air at joints.
- Register-to-return temperature difference under 12°F means a real problem.
What the tech checks
- Static pressure — the best duct/blower indicator.
- Evaporator coil inspection.
- Charge by superheat/subcooling.
- Duct leakage assessment.
Typical: evaporator coil cleaning $150–$450 · blower motor $400–$1,200 · duct sealing $300–$1,500 · charge correction $200–$500.
Leaking water inside
puddle under the air handler
Likely causes, most common first
- Clogged condensate line. Algae. The large majority of indoor leaks, especially Houston and Miami.
- Cracked or rusted pan.
- Frozen coil thawing. Overwhelms the drain.
- Mis-sloped or disconnected drain.
- Failed condensate pump.
Check before you call
- Shut the system off — water damage compounds.
- Find the PVC drain stub outside; a wet/dry vac on it clears most clogs.
- Ice on the coil? It's airflow or charge wearing a leak costume.
- Pump running? Float stuck?
What the tech checks
- Drain clearing and flow verification.
- Pan, slope, float switch.
- Underlying airflow/charge fault if ice was involved.
- Pump and check valve.
Typical: drain clearing $75–$250 · pump $150–$350 · pan $250–$700. Water damage is the expensive part — keep it off until fixed.
A noise it didn't make before
identify it by the sound
Likely causes, most common first
- Buzzing, fan not turning. Capacitor.
- Repeated clicking. Contactor chatter or a board relay.
- Grinding or screeching. Motor bearings. Stop running it.
- Hissing or bubbling. Refrigerant escaping.
- Banging on startup. Loose panel, debris in the fan, failing compressor mount.
- Hum, click, repeat. Compressor tripping its overload.
Check before you call
- Locate it: indoor, outdoor, or ducts.
- Buzzing outdoor unit with a still fan — shut it off. Don't push the fan.
- Rattling — check panels and screws first.
- Grinding — off, now.
What the tech checks
- Capacitor, motor amps, bearing play.
- Contactor and relays under load.
- Electronic leak detection.
- Compressor start characteristics.
Typical: capacitor $150–$400 · contactor $150–$350 · condenser fan motor $300–$700 · blower motor $400–$1,200 · hard-start kit $150–$350.
Freezing up
ice on the lines or coil
Likely causes, most common first
- Restricted airflow. Filter, coil, blower.
- Low refrigerant. Lower pressure, colder coil.
- Running cool mode in cold weather. Below ~60°F outdoors most systems ice without a low-ambient kit. Rare in these five metros; happens in LA winters.
- Stuck contactor. Outdoor unit never stops.
- Failed blower. Coil freezes solid within the hour.
Check before you call
- Cooling off, FAN ONLY, 2–4 hours to thaw. Running it frozen slugs the compressor.
- Replace the filter while it thaws.
- Ices again within an hour with a clean filter → charge or blower. Call.
- Don't chip ice off the fins.
What the tech checks
- Airflow first — static, blower, coil.
- Then charge on a fully thawed system.
- Contactor for welded contacts.
- Leak search if charge is low.
Typical: filter, nothing · blower $400–$1,200 · leak + recharge $400–$1,500 · contactor $150–$350.
Stop and call now if
- Burning or electrical smell from a vent or the outdoor unit
- The breaker trips again right after reset
- Buzzing or arcing at the disconnect or contactor
- Ice on the lines while running
- Indoor temperature climbing on a heat-advisory day
- Water spreading beyond the pan toward walls or floor
Heat pump? Most odd behavior is normal
Heat pumps cool like an AC and fail like an AC in summer — everything above applies. They also do things an AC never does. Which of these needs a call:
- Cool air for a few minutes in heating mode
- Defrost cycle. 2–10 minutes, every 30–90 minutes in cold damp weather. Backup heat covers the dip.
- Steam off the outdoor unit in winter
- Frost melting during defrost.
- Runs almost constantly when it's cold
- Capacity drops as it gets colder. Long runs are correct; short cycling would be the concern.
- Stuck in one mode regardless of thermostat
- Reversing valve or its solenoid. The fault unique to heat pumps. $500–$1,200.
- Thick ice on the outdoor coil that never clears
- Defrost board, outdoor sensor, or reversing valve. $250–$600 for board/sensor.
- Heating bill jumped, house not warmer
- System has quietly shifted to backup strips — low charge, failed outdoor fan, or defrost fault.
Normal
Normal
Normal
Call
Call
Call
Not every AC tech works on heat pumps. Say "heat pump" when you call and we route accordingly. Considering replacing one? Heat pump vs furnace vs AC is covered on the installation page.
Repair or replace — the arithmetic
A 15-year-old 10 SEER unit delivers maybe 8 SEER. A current 15 SEER2 unit uses roughly 40–45% less electricity for the same cooling. On a $250/month Phoenix or Houston summer bill that's $100+/month back. Put that against the repair quote and the replacement quote. Also: equipment since 2025 uses R-454B or R-32, not R-410A — a replacement now is on the current refrigerant. Full decision on the installation page.
AC repair by metro
Each city page covers local climate, permits, utility rebates and the failure modes specific to that market.
References: EPA — HFC phasedown and the refrigerant transition · US Department of Energy · Carrier residential · Trane residential
Written and maintained by Adrian, founder of Urgent HVAC Service — placing HVAC calls across the US since 2016. Last reviewed September 2026. Updated when the content changes, not on a schedule.
Ask before you call
Not sure what's wrong, or whether it's urgent? Describe it. Questions that come up often get added to this page.